1. Field of the Invention
The present invention relates to an exposure control mechanism for use in an image pickup apparatus in which a diaphragm aperture formed by a plurality of diaphragm blades moving straightforward in opposite directions is covered by an ND filter, and more particularly to the technique of suppressing deterioration of image quality caused by diffraction even in an image pickup device having a small picture size and a short pixel pitch.
2. Description of the Related Art
In image pickup apparatuses such as video cameras, an exposure control mechanism comprising two diaphragm blades moved on a straight line in opposite directions for reduction of size, weight and cost have become more commonly used instead of the so-called “iris diaphragm” wherein a plurality of diaphragm blades are rotated about an optical axis to adjust the aperture size.
However, if the aperture size becomes too small when a subject is bright, there occur such problems that image quality is deteriorated due to diffraction and smudges are brought into an image due to an increase of the depth of focus.
In view of those problems, a mechanism has been proposed in which an ND filter is affixed to one of diaphragm blades in such a manner as to project into a cutout which is formed in the diaphragm blade to define the aperture size, so that the aperture is avoided from becoming extremely small.
FIG. 14 shows one example of an exposure control mechanism for use in conventional image pickup apparatuses.
An exposure control mechanism a comprises two diaphragm blades b, c and a drive means d for driving the diaphragm blades b, c.
One (front) diaphragm blade b has a cutout e formed at a lower end for defining the aperture size. Two guided slits f, f extending vertically are formed in the diaphragm blade b at a position near a right edge thereof in vertically spaced relation. Also, a guided slit: g extending vertically is formed in the diaphragm blade b at a position near a left edge thereof.
It is to be noted that directions U, D, L, R, F and B indicated by arrows in the drawings, including FIG. 14, represent the upward, downward, leftward, rightward, forward and backward directions, respectively.
A connecting slot h being elongate horizontally is formed in the diaphragm blade b at a position just above the upper right guided slit f.
Guide pins provided on a housing (not shown), which has formed therein a light passing hole, are slidably engaged in the guided slits f, f and g, respectively. The diaphragm blade b is thereby supported by the housing in a vertically slidable manner.
The other (rear) diaphragm blade c has a cutout i formed at an upper end for defining the aperture size, and an ND filter j is attached to the diaphragm blade c so as to cover a lower end area of the aperture size defining cutout i. Two guided slits k, k extending vertically are formed in the diaphragm blade c at a position near a left edge thereof in vertically spaced relation. Also, a guided slit 1 extending vertically is formed in the diaphragm blade b at a position near a right edge thereof. Incidentally, the ND filter j has transmittance of 10%.
Furthermore, a connecting slot m being elongate horizontally is formed in the diaphragm blade c at a position just above the upper left guided slit k.
Guide pins provided on the housing (not shown) are slidably engaged in the guided slits k, k and l, respectively. The diaphragm blade c is thereby supported by the housing in a vertically slidable manner.
The drive means d comprises a drive motor n attached to an upper portion of the housing (not shown), and an operating arm o fixed to a rotary shaft of the drive motor n.
The operating arm o extends substantially in the right-and-left direction, and is fixed at its central portion to the rotary shaft of the drive motor n. Also, connecting pins p, p are projected respectively from right and left ends of the operating arm o.
The connecting pin p at the right end of the operating arm o is slidably engaged in the connecting slot h of the diaphragm blade b, and the connecting pin p at the left end of the operating arm o is slidably engaged in the connecting slot m of the diaphragm blade c.
Accordingly, when the operating arm o is rotated by energizing the drive motor n, the connecting pins p, p are moved in opposite directions, whereupon the diaphragm blades b, c coupled to the connecting pins p, p are moved vertically in opposite directions. As a result, a diaphragm aperture q defined by the aperture size defining cutouts e, i of the two diaphragm blades b, c is changed.
FIGS. 15a to 15f show a manner in which the ND filter j covers the diaphragm aperture q when the diaphragm aperture q is gradually narrowed from an open state (FIG. 15a) to a small aperture state (FIG. 15f) by moving the diaphragm blades b, c of the exposure control mechanism a.
FIG. 16 shows values of an MTF (modulation transfer function) depending on various sizes of the diaphragm aperture q indicated in FIGS. 15a to 15f. Here, the MTF value means a diffraction limit value of the white MTF value determined by calculating, based on wave optics, the capability in the vertical direction (line image in the horizontal direction) at spatial frequency corresponding to the TV resolution of about 260 lines. Also, the dimension of the ND filter j is decided so that the diaphragm aperture q has a size corresponding to F 5.6 at the moment when the diaphragm aperture q is entirely covered by the ND filter j (see FIG. 15e). The MTF value at that moment is 0.73.
Specifically, the MTF value means a diffraction limit value of the white MTF value determined by calculating, based on wave optics, the capability in the vertical direction (line image in the horizontal direction) that is evaluated by the fact that the effect of diffraction appears significantly in the states of FIGS. 15a to 15f, in view of spatial frequency of 48 lines/mm corresponding to the TV resolution of about 260 lines, i.e., frequency representing image quality in a motion video camera comprising an image pickup device wherein the picture diagonal length is 4.5 mm, a pixel pitch is about 5 μm, and the number of effective pixels is 380,000.
Accordingly, deterioration of image quality is regarded as being allowed if the MTF value is not less than a predetermined value. The MTF value =0.65 has been employed, by way of example, as an allowable limit value in the past. Note that the MTF value is not an absolute value, but a relative value used for determining whether deterioration of image quality is in the allowable range.
In the case of conventional image pickup devices in which the picture diagonal length is 4.5 mm, as shown by a solid line in FIG. 16, when the diaphragm blades b, c are moved to gradually narrow the diaphragm aperture q, the MTF value is also gradually reduced, and takes a minimum value in the state shown in FIG. 16d, i.e., at the aperture size d. Then, the MTF value increases again and takes a maximum value in the state shown in FIG. 16e, i.e., at the aperture size e. Thereafter, the MTF value decreases again.
The reason why the MTF value takes a minimum value at the aperture size d is that a vacant space area surrounded by the diaphragm blade b and the ND filter j serves as like a small aperture to develop diffraction, and image quality is deteriorated in an intermediate aperture state.
When the diaphragm blades b, c are further moved to gradually narrow the diaphragm aperture q, the MTF value increases again and takes a maximum value at the aperture size e. This is because until the diaphragm aperture q changes from the aperture size d to the aperture size e at which the diaphragm aperture q is completely covered by the ND filter j, the effect of diffraction is gradually reduced so that the MTF value increases. When the diaphragm aperture q is further narrowed from the aperture size e, the MTF value decreases again due to the effect of diffraction.
Taking into account such changes of the MTF value, it has been customary that the transmittance of the ND filter j is designed to keep the MTF value not less than 0.65 in all the sates from the open aperture a to the small aperture f.
Recently, in image pickup apparatuses, there has been a tendency to reduce the picture size of an image pickup device. A reduction in the picture size of the image pickup device decreases the pixel pitch and increases the effect of diffraction, thus making it hard to obtain satisfactory image quality. While conventional image pickup devices had a picture diagonal length of 4.5 mm, for example, the picture diagonal length has been recently reduced to 2.25 mm. The spatial frequency corresponding to the TV resolution of about 260 lines is 48 lines/mm for the picture diagonal length of 4.5 mm, and 96 lines/mm for the picture diagonal length of 2.25 mm. If the picture diagonal length of an image pickup device is changed to 4.5 mm with the pixel pitch remained unchanged, this case corresponds to frequency representing image quality in a still-video camera comprising 1.5 millions pixels.
When an image pickup device having a picture diagonal length of 2.25 mm is employed in combination with the conventional exposure control mechanism a, the spatial frequency is doubled and therefore the effect of diffraction is remarkably increased, thus giving rise to a problem that image quality is deteriorated.
More specifically, a broken line in FIG. 16 shows a curve of the MTF value resulted when the conventional exposure control mechanism a is combined with such a small image pickup device. As seen, the MTF value is reduced down below 0.65 in a state in which the diaphragm aperture q is narrowed from the aperture size b to some extent. This means that the above combination is not practicable. In other words, the conventional exposure control mechanism a has a problem of being not adaptable for downsizing of the image pickup device.